SIT is widely accepted as a useful treatment of allergic disease despite an incomplete knowledge of mechanism of action and variability in efficacy. This project is focused on understanding mechanisms behind SIT and specifically tests the hypothesis that certain allergens, antigens, and epitopes are more prominently or potentially differentially recognized by SIT treated donors as compared to untreated allergic individuals. We further test the related hypothesis that responses to certain novel anfigens, recently identified by our group, may play a role in the successful outcome of SIT treatment. By characterizing the functional phenotype ofthe T cell responses to the known and novel antigens alike, we will identify changes occurring as a result of SIT. We will also test two specific hypotheses on mechanisms associated with SIT, namely: 1) SIT modulates Tfh cells to promote a shift from IgE to IgG production and that 2) SIT induces Tri cells with suppressive regulatory capacity. Previous work from our group in the Timothy grass (TG) system delineated the major epitopes associated with ten known major allergens in allergic and SIT treated donors. In those major TG allergens, we did not identify major targets of ILIO producing T cells. Recent data from our group using a transcriptome and proteome analysis of TG pollen and 2D gel immunoblots with IgE and IgG antibodies from SIT and allergic donors has led to the identification of several novel antigens that are selectively recognized by antibodies from SIT individuals, induce IgG (but not IgE) responses, and others that do not induce an antibody response. Several of these novel antigens are vigorously recognized by T cells, particularly from PBMC from SIT donors. Our studies will test the hypothesis that qualitative shifts in response specificity may be important in inducing efficacy of SIT. Epitopes from both known and novel antigens will be used in longitudinal studies, monitoring magnitude and phenotype of responding T cells. If successful, the proposed studies will highlight potentially useful novel immunogens for use in place of current SIT that may be safer and more effective. In addition, our studies may identify biomarkers that indicate successful SIT treatment.